1. Field of the Invention
The invention relates to an electric power steering system and a vehicle steering system.
2. Description of Related Art
There is an electric power steering system (EPS) for a vehicle, which includes a first pinion shaft that is rotated in response to an operation of a steering wheel and a second pinion shaft that is rotated by a motor. In such an electric power steering system, the first pinion shaft and the second pinion shaft are in mesh with a rack shaft, and the torque of the motor is applied to a steering system as assist force.
Japanese Utility Model Application Publication No. 60-36374 describes an electric power steering system in which a rack shaft has two racks that are formed on the axially opposite sides of the axial center of the rack shaft, and pinion shafts are in mesh with the respective racks.
Many in-vehicle components, such as an engine and a radiator, are concentrated in the lateral center area of an engine compartment. If the above-described structure is employed, contact between these in-vehicle components and a motor of the electric power steering system is less likely to occur, which makes installation of these components easier. In the example described above, the two racks differ from each other in pitch (interval between consecutive rack teeth). With this structure, the motor load is reduced.
In the meantime, in a rack-and-pinion mechanism constituted of a rack shaft and a pinion shaft, force that causes the rack shaft and the pinion shaft to move away from each other is produced by the meshing friction. Therefore, usually, the rack shaft is pushed against the pinion shaft by a known rack guide so that the rack shaft is kept in mesh with the pinion shaft. With regard to such a structure, refer to Japanese Patent Application Publication No. 06-92243, for example.
However, when two rack-and-pinion mechanisms are provided at one rack shaft as described above, the forces acting on the meshing points of the respective rack-and-pinion mechanisms may amplify each other. The vibrations that occur due to such force amplification may reduce the quietness of the operation of the electric power steering system.
As in the example case described above, if the two racks differ from each other in pitch, the state of gear teeth engagement (e.g., the positional relation between the tip of the tooth currently in engagement and the tooth bottom mating with this tooth tip, and the gear teeth engagement timing) between the rack and the pinion shaft of one of the two rack-and-pinion mechanism is not synchronous with that of the other rack-and-pinion mechanism. This prevents the vibrations caused by gear teeth engagements from continuously kept at high levels. However, the gear teeth engagement states of the two rack-and-pinion mechanisms may coincide, although depending upon the axial positions of the pinion shafts relative to the respective racks, with each other, thereby creating a state where the forces acting on the meshing points of the respective rack-and-pinion mechanisms both peak. Further, because the vibration level changes in accordance with the current steering angle within the entire steering angle range, there is a possibility that an occupant of a vehicle may easily feel the vibrations.